This invention relates to braking devices for an elevator wherein the riding comfort at the stopping periods, especially the riding comfort at the stopping times during the low speed inspection operations, is improved.
FIG. 1 is a schematic side view of a conventional electromagnetic braking device which is assembled integrally with the winding machine. FIG. 1 shows the state in which the movement of the brake wheel is arrested by the brake shoes. Thus, brake levers 50 are urged by springs 51 in the directions A, so that brake shoes 52 attached to the brake levers 50 bear on the brake wheel 53 to arrest the rotation thereof. The brake wheel 53 is fixedly mounted to the driving shaft 54 directly coupled to the electric motor (not shown) for driving the elevator cages; thus, the brake wheel 53 arrests the rotation of the motor and hence the operation of the elevator cages. In this state, L-formed cams 55 are rotated in the directions B, accompanying the movements of the brake levers 50 in the directions A, so as to push the plunger 56 upward.
When a current is supplied from a power source (not shown) to a brake coil 57 disposed around the plunger 56, the plunger 56 is attracted downward. Accompanying the downward movement of the plunger, the cams 55 are rotated in the directions C, so as to rotate the brake levers 50 in the directions of the arrows D. Accompanying these rotations, the brake shoes 52 release the brake wheel 53. As a result, the shaft 54 set free to be driven by the motor, to raise and lower the elevator cages.
Assuming that the air gap of the magnetic circuit at the plunger 56 is represented by x (see FIG. 1), the electromagnetic attraction force FP resulting from a current i flowing through the brake coil 57 is expressed approximately by the following equation: EQU FP=K1.multidot.(i.sup.2 /x.sup.2) (1)
where K1 is a constant of proportionality. On the other hand, the force FB by which the springs 51 urge the plunger 56 via the levers 55 and the cams 50 is also a function of the air gap x, which can be expressed by the following equation: EQU FB=F.sub.0 -K2.multidot.x (2)
wherein F.sub.0 is the urging force of the springs 51 when x is equal to zero, and K2is a constant of proportionality.
When the current i is fixed, the relations of FP and FB to the air gap x can be shown by a curve as shown in FIG. 2. In FIG. 2, the point a on the abscissa represents the magnitude of the air gap x at the position where the plunger 56 is attracted downward by the coil 57. In the gap between the points O and a, there is inserted a washer (not shown in FIG. 1) usually made of leather, for the purpose of quickening the operation of the plunger 56 and reducing the operation noises thereof. Thus, the point a is the position of the end of the plunger when the plunger 56 is attracted downward and the brake is thus released. The point b represents the position of the end of the plunger 56 when the brake shoes 52 are bearing on the brake wheel 53 to arrest the rotation thereof. The line FB represents the resilient force by which the plunger 56 is urged by the springs 51 at respective air gaps x. The curves FP1 through FP3 represent the electromagnetic attractions resulting from the braking coil currents i1 through i3, respectively.
Let us assume that the end of the plulnger 56 is at the point b during an elevator stop period; further assume that the brake coil excitation current increases to reach i1; then, the electromagnetic attraction acting on the plunger 56 overcomes the resilient force by which the plunger 56 is urged, so that the plunger 56 moves toward left in FIG. 2 (in the decreasing direction of the air gap x). The smaller the air gap x becomes, the stronger becomes the electromagnetic attraction of the plunger 56, as shown by the curve i1 in FIG. 2, the end of the plunger 56 thus moving rapidly to the point a. At the point a, the electromagnetic attraction acting on the plunger 56 is equal to a-c, which is sufficiently great with respect to the resilient force a-d, so that the electromagnetic brake is maintained stably in the released state. On the other hand, when the rotation of the driving shaft 54 of the elevator is to be arrested by the elelctromagnetic brake to stop the elevator cages, the current is decreased from i1 to i3. Then, the electromagnetic attraction acting on the plunger 56 is overcome by the resilient force of the springs, the plunger being thereby translated toward the right in FIG. 2 (in the increasing direciton of the air gap x). As the air gap x increases, the electromagnetic attraction becomes weaker, as shown by the curve i3 in FIG. 2, the end of the plunger 56 thus moving rapidly to the point b. At the point b, the braking force is approximately proportional to the resilient force b-f, since the electromagnetic force b-e counteracting it is small. Thus, even when the braking coil excitation current is decreased as slowly as possible, the braking force abruptly becomes approximately proportional to the resilient force b-f, so that the stops in low speed operations during inspections of the elevator or during imergency stops due to failures, tend to be abrupt.
The stopping operations during the usual automatic operations are effected, except in emergencies, by means of electric controls of the main motor for driving the elevator cages. The riding comfort during such automatic operations has been greatly improved thanks to the enhanced control performance realized by the recent advanced control technology. However, in spite of these recent advances, abrupt deceleration at the stops during the low speed operations is becoming even greater, because, first, the inertia of the motor shaft is becoming increasingly smaller, and second, it is necessary that the braking torque has a magnitude which overcomes the torque unbalance that may be caused under the overload condition. Thus, the braking torque cannot be made smaller than a predetermined magnitude. Consequently, the abrupt stops during low speed inspection operations of elevator cages have been inevitable. This not only results in the increased physical fatigue of the installment/inspection operators of the elevator, but also in safety problems of such operations.